The field of the present invention relates to ophthalmic lenses, and particularly to an ophthalmic lens utilizing a polarizing film.
A variety of ophthalmic lenses are known as described in, for example, U.S. Pat. Nos. 4,418,992 and 4,274,717, each of these disclosures hereby incorporated by reference as if fully set forth herein. Such lenses may comprise a number of different types of materials ranging from inorganic to thermoset plastics, such as allyl diglycol carbonate sold under the CR-39.RTM. trademark of PPG Industries, Inc. ("CR-39.RTM."), to more recent formulations using thermoplastic materials, such as polycarbonate ("PC").
Commonly, polarizers used in hard resin thermoset lenses or polycarbonate thermoplastic lenses are based on polyvinylalcohol ("PVA") films imbued with a polarizing material. For thermoset lenses, the polarizing film is either adhesively bonded to a lens substrate, or it is placed within a mold assembly and the liquid resin mixture placed around it (sequentially or simultaneously) to form the lens. For thermoplastic lens production, the film is commonly part of a multi-layer construction (often referred to as a wafer) designed for better rigidity and thermal stability. Often this construction involves joining or encapsulating the polarizer with other polymers such as PC or cellulose acetate butyrate ("CAB") by co-extrusion, lamination, calendering, etc.
There are several limitations with these approaches. The common PVA base film is temperature-sensitive and therefore difficult to process with thermoplastics. In thermoplastic lens manufacturing, for example, monomer or polymeric pellets are heated past their softening point (for PC, above 230.degree. C.), and injected into a mold form. Conventional polarizer films comprising PVA or similar polymers cannot withstand these temperatures. For instance, PVA has its glass transition temperature ("T.sub.g ") between 90-95.degree. C., and softens with decomposition at approximately 200.degree. C. Therefore, not only will the PVA film lose its shape, but it will also lose physical integrity (color, polarization efficiency, mechanical strength, etc.) at typical molding temperatures.
In addition to the temperature-sensitive film, the dyes or polarizing agents commonly used therein are also temperature-sensitive. The temperature-sensitivity of the common polarized film and the dyes or polarizing agents used therein can cause severe non-uniformity or non-reproducibility, adversely affecting either the optical and cosmetic quality of a given lens or lot-to-lot consistency.
The conventional approach to solve the temperature-sensitivity limitation of common polarizers has consistently been to clad the weak polarizing film with another more resistant plastic to survive the molding process. Such cladding is typically done by adhesively joining the film with at least one support layer or interposing the film between layers and adhesively combining the film to the support layers, as described for example in U.S. Pat. No. 5,051,309, which is incorporated by reference as if fully set forth herein.
Common adhesively joined products, however, are susceptible to delamination during either the lens molding process (heat and pressure excursions) or subsequent processing to form finished ophthalmic eyewear. Such delamination is a problem for thermoplastic molding, as well as thermoset resin lenses that sometimes depend upon adhesives to join the polarizer to the solid plastic lens substrate.
One reference, U.S. Pat. No. 5,059,356, which is incorporated by reference as if fully set forth herein, discloses a polarized film of polyethylene terephthalate (PET). This material has several advantages over PVA, including affordability, significantly better heat, moisture, and solvent resistance, and good mechanical stability. It is also more stable to ultra-violet radiation (UV), which is especially useful for lens production and coating processes with UV curing cycles.
The present inventors recognized that an ophthalmic lens utilizing PET film might offer advantages over an ophthalmic lens utilizing current PVA film if the PET film could be reliably incorporated into ophthalmic lens manufacture. Despite the potential advantages, the present inventors are unaware of any effective use of PET film in ophthalmic lenses. Its absence of effective incorporation into ophthalmic lens manufacture can be attributed to a number of different reasons.
The primary reason is due to PET's chemical inertness. A secondary reason includes PET's relatively low degree of optical quality, which is not able to match or accommodate ophthalmic lenses.
With respect to its chemical inertness, the present inventors have experimented with PET film and noted that PET cannot be reliably adhered within or to the lens and additional coatings cannot be reliably adhered to the PET film. In particular, the present inventors discovered that simple adhesive bonding (application of liquid adhesives or glues) is not acceptable for ophthalmic lenses that will undergo additional processing, such as grinding to prescription strength or edging to fit frames, or which will be subjected to additional coating processes for mechanical or optical enhancement.
In addition, it is not obvious that an unsupported PET film could be incorporated in an ophthalmic lens. With thermoset resins, many years of development were required to progress from clad PVA assemblies to free-standing film. While the inventors could build on this development, the challenge of PET's inertness still needed to be overcome to ensure reliable adhesion. With thermoplastics, even greater challenges are present, due to the high temperature processing. The standard approach has been to clad or encapsulate polarizing films. However, additional interfaces between solid surfaces always offer prime sites for delamination during further processing.
Accordingly, an ophthalmic lens and method or process of utilizing PET film to form the lens, wherein the film may be bonded to the lens material without additional plastic supports, without losing its required physical properties, and with high adhesion, is desired.